Add Useful Properties, Deprecate getters

[Teque5]

I think it's a bit silly to read sample rate by doing meta.get_global_field('core:sample_rate').

Right now the only getter that is implemented is get_num_channels().

I think we should add properties for all global fields including sample_rate, datatype, num_channels, et cetera.

And deprecate the getter get_num_channels().